Micro-optical electromechanical system (MOEMS) membranes are used in a spectrum of optical applications. For example, they can be coated to be reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser cavity, for example. They can be used as tip/tip-tilt mirrors in switches.
The MOEMS membranes are typically produced by depositing a membrane structure over a sacrificial layer, which has been deposited on a support structure. This sacrificial layer is subsequently etched away to produce a suspended membrane structure in a release process. Often the membrane layer is a silicon compound and the sacrificial layer can be polyimide, for example.
Typically, membrane deflection is achieved by applying a voltage between the membrane and a fixed electrode on the support structure. Electrostatic attraction moves the membrane in the direction of the fixed electrode as a function of the applied voltage. This results in changes in the reflector separation of the FP filter or cavity length in the case of a laser.
In typical operation in an FP filter, a mode within an order of operation of the tunable filter is scanned across some spectral band of interest. For example, in dense wavelength division multiplexing (DWDM) systems such as defined by the current ITU (International Telecommunications Union) grid, channel slots are defined between approximately 1490 nanometers (nm) and 1620 nm, for the L, C, and S bands. If the filter were designed to scan, for example, the C band, it would be desirable to place a mode of the filter at 1570 nm at a zero Volts condition, for example, and then scan that mode by deflecting the membrane to approximately 1530 nm by ramping the electrostatic drive voltage. Similarly, if a full L, C and S band scan is to be performed, it would be desirable to spectrally place the mode of the filter at approximately 1490 nm for zero Volts and then scan it to approximately 1611 nm by ramping the voltage.
For reasons associated with MOEMS filter fabrication, however, the operation of the tunable filter is slightly more complex. The spacing between orders of operation for a filter is termed the filter""s free spectral range (FSR). This FSR is typically determined by a spacer layer(s) between the optical membrane and stationary reflector. The location of the filter""s modes typically depends upon the curvature of the reflectors of the FP cavity. In order to determine the location of the filter passband at rest, when there is no electrostatic field in the cavity, both the membrane/reflector spacing and reflector curvatures need to be specified to high levels of accuracy, for example, within a few nanometers.
Another related issue concerns the physical distance over which the membrane can be deflected. There are typically limitations in the voltages that are available to electrostatically deflect the membrane. For example, many times the systems are designed to run only on a few Volts. Moreover, there can be limitations associated with the stability of the membrane. For example, general electrostatic deflection cavity design parameters specify that a membrane should typically only be deflected over approximately one third of the cavity width to avoid unstable operation.
The present invention concerns a MOEMS Fabry-Perot tunable filter. As is common, it includes an optical membrane structure. Two electrostatic cavities are provided, however, one on either side of the membrane structure. As a result, electrostatic attractive forces can be exerted on the optical membrane to enable deflection in either direction, typically, along the optical axis. This is useful in calibrating the tunable filter during operation to a desired initial wavelength (xcexo) set point. It is also useful in controlling the membrane to avoid unstable operation.
In general, according to one aspect, the invention concerns a triple electrode MOEMS Fabry-Perot tunable filter. It comprises an optical membrane, including a membrane electrode. A first stationary electrode supports an electrical field to deflect the optical membrane in a first direction and a second stationary electrode supports an electrical field to deflect the membrane in a second direction.
In one embodiment, the membrane electrode comprises a conductive layer on the optical membrane. In the typical embodiment, the optical membrane structure itself is conductive to thereby function as the membrane electrode, however. This can be achieved in the context of semiconductor-based devices by controlling the doping of the layer that is used to form the optical membrane structure.
In one embodiment, the optical membrane structure is vaulted to form a hemispherical Fabry-Perot cavity. In another embodiment, it is substantially planar to form a flat reflector of a hemispherical Fabry-Perot cavity.
According to other aspects of the present embodiments, the first stationary electrode comprises a conductor layer on a support wafer. Alternatively, in another embodiment, a support wafer structure is rendered conductive to function as a first stationary electrode.
According to another aspect of one of the embodiments, a reflector of a Fabry-Perot filter comprises a dielectric mirror structure that is deposited on a support wafer structure. Alternatively, an optical port can be formed in a support wafer structure and a mirror attached to the support wafer structure, typically with an intervening spacer layer, to define a Fabry-Perot cavity between the mirror and the membrane structure.
In general, according to another aspect, the invention also features a tunable filter. This filter comprises a support wafer structure and an optical membrane structure, including a membrane electrode that is attached to the support wafer structure. A first electrostatic cavity is provided between the support wafer and a proximal side of the optical membrane structure. A second stationary electrode defines a second electrostatic cavity on a distal side of the membrane structure.
In general, according to still another aspect, the invention concerns a triple electrode optical membrane. This membrane comprises an optical membrane structure, including a membrane electrode. First and second stationary electrodes are provided to deflect the membrane in either direction along an axis that is orthogonal to a surface of the membrane.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.